Up until now, Foursquare, the hot location-based service, has only let a handful of developers play around with its API. Starting today, the company is formally unveiling it, hoping that a robust app community will form around their data.

Here’s the way they’re describing it:

The foursquare API enables developers to build applications that interact with the foursquare platform. You can use the API to create new ways to check-in to foursquare or visualize the data generated by the foursquare community. Our API is still a work in progress and we look forward to your feedback and suggestions!

In announcing the API, Foursquare has also created an app gallery to show what’s already been built ontop of the API so far. Included in this gallery are:

Another third-party app just announced is Layar with support for Foursquare. The augmented reality browser has a new beta version with functionality that allows you to use your mobile device to look around and see Foursquare venues around you. You don’t even have to have an account with Foursquare for this to work, so it seems like it could be a good way to convince new users to sign up. If you do have an account, you can checkin to venues right from within Layar.

That’s a pretty good list of third-party support considering that access to the API has been very restricted up until this point. The opening of the data is likely to be important for Foursquare in the long run. Foursquare is getting traction with users putting information into its system, but a real explosion could come if third-party developers start making some great apps on top of the service. Obviously, we’ve seen that type of community swarm around and feed services like Twitter.